The invention is directed to a process for the production of a tantalum solid or dry-electrolytic capacitor comprising a sintered tantalum anode to which an oxide layer, functioning as dielectric, is applied by a forming process, and which is provided with a semiconducting manganese dioxide layer, functioning as cathode, which, in turn, is coated with a graphite layer functioning as current supply.
The quality of electrolytic capacitors is not determined solely by the properties of the dielectric, but to a high degree also by the efficiency of the cathode supply line. In a tantalum dry-electrolytic capacitor, semiconducting manganese dioxide is employed as the cathode. In the production of such cathode, the formed sintered anoded is temporarily submerged in an aqueous manganese-nitrate solution with the manganese nitrate subsequently being pyrolytically transformed into manganese dioxide. The coating process is repeated several times to achieve a highly adhesive and electrically satisfactory layer of manganese dioxide. In addition, after several pyrolysis operations, an intermediate forming process is interposed. Finally, a thin graphite layer is produced on the finished manganese dioxide layer by submerging the structure in a solution with colloidal graphite. Such thin graphite layer functions as current supply to the cathode and, if desired or required, can be suitably contacted, for example with a conductive silver lacquer layer.
The present invention is based upon the recognition that the series resistance in a tantalum dry-electrolytic capacitor, and thus also the loss factor and apparent impedance, is not determined solely by the sum of the bulk resistances of the individual layers, but that the conditions at the boundary area between the manganese dioxide and graphite layers also exert a considerable influence.